The present invention relates to a communication radio channel assignment control of a mobile communication system which commonly uses a single frequency in a plurality of radio zones and, more particularly, to a method of selecting a communication radio channel having high communication quality when interference is caused from other communication zones.
In a known conventional mobile communication system, a radio zone capable of effectively performing communication with a mobile station is set about a single base station, and the base station and the mobile station in the radio zone perform communication by designating a communication radio channel. When a traffic density is high and a large number of communication channels are present, a control radio channel for selecting a communication radio channel is often provided in addition to the communication radio channels.
In order to effectively use a radio frequency, a mobile communication system for servicing a wide area adopts a small zone system in which a radio zone is divided into small zones and a single radio frequency is locally repeatedly used (a range to which a radio wave from a base station reaches is called a zone, and a base station arranging method and a method of repeatedly using a frequency are called a zone arranging method).
In general, when a radio frequency band (the number of radio channels) given to a system is constant, a subscriber capacity is increased as the area of a repetitive area is decreased. In an urban area having a high traffic density, therefore, a zone must be densely arranged as small as possible in order to effectively use a frequency. As the density of a zone is increased, a repetitive use distance of a single radio frequency is shortened to increase the subscriber capacity. In this case, interference from the same radio frequency in a peripheral region can be theoretically so controlled as not to be increased. Actually, however, interference of the same channel is increased upon repetitive use due to an influence of configuration of the ground or natural feature on the earth, thereby significantly degrading communication quality.
FIG. 1 is a view for explaining interference in a base station and a mobile station. Referring to FIG. 1, reference numeral 51 denotes a radio wave of a communication radio channel; 52 and 53, jamming waves from communication radio channels having the same frequency; 54, a base station; 55, a mobile station; and 56 and 57, radio zones.
The base station 54 receives the jamming wave 52 in addition to the radio wave 51 of the communication radio channel. This jamming wave 52 functions as an interference wave. The mobile station 55 receives the jamming wave 53 in addition to the radio wave 51 of the communication radio channel, and this jamming wave 53 functions as an interference wave. As is apparent from FIG. 1, a propagation path of interference caused by the jamming wave 52 is different from that of interference caused by the jamming wave 53. That is, interference conditions are generally different between a base station and a mobile station.
As described above, interference is easily caused when a single frequency is commonly used in a plurality of radio zones and, especially when repetitive use is densely performed in order to increase a use efficiency of the frequency.
As a prior art, a method of detecting, upon selection of a communication channel, the presence of a radio wave received by a communication channel in a base station and selecting a communication channel free from radio wave interference in the base station is available (Japanese Patent Laid-Open No. 53-28311). In general, however, since interference conditions are different between a base station and a mobile station, interference cannot be prevented from being caused at a mobile station side when a communication channel is selected in a base station.
In addition, a method of measuring a reception level of an interference wave when a communication channel is designated for a mobile station and a base station and performing channel switching in a radio zone when a degree of interference is high has been adopted.
This channel switching method, however, cannot be a suitable method since its control load is comparatively high and radio link hit or noise mixing occurs to degrade communication quality.
In addition, the method has another problem in which when an assigned communication radio channel is interfered immediately after assignment, predetermined quality cannot be obtained until channel switching is performed in a zone.